cachedb_redis: add dynamic cluster topology management and observability

Replace the static cluster topology (built once at startup, never
refreshed) with runtime discovery and automatic refresh:

Topology discovery and refresh:
- Probe CLUSTER SHARDS (Redis 7+) with fallback to CLUSTER SLOTS
  (Redis 3+) for backward compatibility
- O(1) slot_table[16384] lookup replaces per-query linked-list scan
- Automatic topology refresh on MOVED redirect, connection failure,
  or query targeting an unmapped slot (rate-limited to 1/sec)
- Dynamic node creation when MOVED points to an unknown endpoint
- Stale node pruning during refresh with safe connection cleanup
- Cap redirect loop at 5 max redirects to prevent worker hang on
  pathological cluster state

Cluster observability via MI commands:
- redis_cluster_info: full topology dump including per-node connection
  status, slot assignments, query/error/moved/ask counters, and
  last activity timestamp
- redis_cluster_refresh: trigger manual topology refresh (bypasses
  rate limit)
- redis_ping_nodes: per-node PING with microsecond latency reporting
- All MI commands support optional group filter parameter

Statistics:
- redis_queries, redis_queries_failed, redis_moved, redis_ask,
  redis_topology_refreshes (module-level stat counters)
- Per-node query, error, moved, ask counters in redis_cluster_info

Hash slot correctness:
- Hash tag {…} extraction per Redis Cluster specification
- CRC16 modulo 16384 replaces bitwise AND with slots_assigned

ASK redirect handling:
- Detect ASK responses alongside existing MOVED handling
- Send ASKING command to target node before retrying original query
- Do not update slot map (ASK is a temporary mid-migration redirect)
- Refactor parse_moved_reply into parse_redirect_reply with prefix
  parameter; inline wrappers for backward compatibility

Connection reliability:
- TCP keepalive via redis_keepalive parameter (default 10s)
- Stack allocation for redis_moved structs (eliminates OOM paths)
- NULL guards on malformed CLUSTER SHARDS/SLOTS reply elements
- Integer overflow protection in slot and port parsing
- NULL guards in MI command handlers for group_name/initial_url

Documentation:
- New section: Redis Cluster Support (topology discovery, automatic
  refresh, MOVED/ASK handling, hash tags)
- MI command reference: redis_cluster_info, redis_cluster_refresh,
  redis_ping_nodes
- Authentication URL format documentation (classic, ACL, no-auth)
- New parameter: redis_keepalive

Test suite (186 tests):
- C unit tests: hash slot calculation (37), MI counter helpers (41)
- Integration: topology startup (12), ASK redirect (16), topology
  refresh (13), MI commands (50), edge cases (16)
- Trap EXIT handlers for safe cluster state restoration
- python3 preflight checks for JSON-dependent tests

Depends on: #3815 (hash tag + modulo fix), #3852 (ASK redirect)

Author: Debian

modules/cachedb_redis/README

@@ -39,12 +39,14 @@
 #include "../../cachedb/cachedb.h"
 
 #include "cachedb_redis_dbase.h"
+#include "cachedb_redis_mi.h"
+#include "../../statistics.h"
 
 static int mod_init(void);
 static int child_init(int);
 static void destroy(void);
 
-static str cache_mod_name = str_init("redis");
+str cache_mod_name = str_init("redis");
 struct cachedb_url *redis_script_urls = NULL;
 
 int set_connection(unsigned int type, void *val)
@@ -62,6 +64,7 @@ static const param_export_t params[]={
 	{ "ftsearch_json_prefix",        STR_PARAM,                &fts_json_prefix.s},
 	{ "ftsearch_max_results",        INT_PARAM,                &fts_max_results},
 	{ "ftsearch_json_mset_expire",   INT_PARAM,                &fts_json_mset_expire},
+	{ "redis_keepalive",             INT_PARAM,                &redis_keepalive},
 	{0,0,0}
 };
 
@@ -92,6 +95,33 @@ static const dep_export_t deps = {
 	},
 };
 
+static const stat_export_t mod_stats[] = {
+	{"redis_queries",            0, &redis_stat_queries          },
+	{"redis_queries_failed",     0, &redis_stat_queries_failed   },
+	{"redis_moved",              0, &redis_stat_moved            },
+	{"redis_topology_refreshes", 0, &redis_stat_topology_refreshes},
+	{0, 0, 0}
+};
+
+static const mi_export_t mi_cmds[] = {
+	{ MI_REDIS_CLUSTER_INFO, 0, MI_NAMED_PARAMS_ONLY, 0, {
+		{mi_redis_cluster_info, {0}},
+		{mi_redis_cluster_info_1, {"group", 0}},
+		{EMPTY_MI_RECIPE}}, {0}
+	},
+	{ MI_REDIS_CLUSTER_REFRESH, 0, MI_NAMED_PARAMS_ONLY, 0, {
+		{mi_redis_cluster_refresh, {0}},
+		{mi_redis_cluster_refresh_1, {"group", 0}},
+		{EMPTY_MI_RECIPE}}, {0}
+	},
+	{ MI_REDIS_PING_NODES, 0, MI_NAMED_PARAMS_ONLY, 0, {
+		{mi_redis_ping_nodes, {0}},
+		{mi_redis_ping_nodes_1, {"group", 0}},
+		{EMPTY_MI_RECIPE}}, {0}
+	},
+	{EMPTY_MI_EXPORT}
+};
+
 /** module exports */
 struct module_exports exports= {
 	"cachedb_redis",					/* module name */
@@ -103,8 +133,8 @@ struct module_exports exports= {
 	0,						/* exported functions */
 	0,						/* exported async functions */
 	params,						/* exported parameters */
-	0,							/* exported statistics */
-	0,							/* exported MI functions */
+	mod_stats,					/* exported statistics */
+	mi_cmds,					/* exported MI functions */
 	0,							/* exported pseudo-variables */
 	0,							/* exported transformations */
 	0,							/* extra processes */